When drilling a wellbore, a drill bit is generally attached to the bottom of a drill string. For some wellbores, especially highly deviated or horizontal wells, rather than rotating the entire drill string from the surface, a motor located near the drill bit may be used to rotate the drill bit and, in some instances, power downhole equipment. In some downhole motors, a progressing cavity pump (also known as a progressive cavity pump, eccentric screw pump, cavity pump, or Moineau pump) may be utilized to, as drilling fluids are pumped therethrough, rotate the drill bit. As understood in the art, a progressing cavity pump is a positive displacement pump which includes a stator and rotor. The rotor has one or more generally helical outer profiles extending down its length. The stator has two or more generally helical inner profiles extending down its length. The helical outer profile of the rotor and the helical inner profile of the stator are configured to allow, as the rotor eccentrically rotates within the stator, form a series of cavities down the length of the progressing cavity pump. When the rotor is driven, the progressing cavity pump serves to pump a fluid. Alternatively, by externally pumping a fluid through the progressing cavity pump, the rotor of the pump rotates, allowing rotational power to be extracted from the flow. When used as part of a drilling string, such motors, commonly known as drilling or mud motors, utilize the flow of a drilling fluid through the drill string to rotate downhole equipment, sometimes including the drill bit.
In many progressing cavity pumps, the stator includes an outer cover and inner elastomeric layer which is molded between the interior of the outer cover and a core, the core being a negative mold of the desired profile of the interior of the stator. The use of the elastomeric layer may serve to simplify construction of the stator as well as allow for reactive movement of the interior profile of the stator to, for example, create an interference fit against the rotor or to allow relatively large solid particles (including, for example and without limitation, lost circulation material) to pass therethrough while minimizing damage to the stator. Generally, the tighter the fit between the stator and rotor, the more power can be extracted from the movement of drilling fluid therethrough. However, depending on the temperature, content of fluid passing through the progressing cavity pump including solid particles or abrasive or corrosive compounds, swelling or contracting of pump components, and the fit between the stator and rotor, the elastomeric layer may be eroded or worn.
Additionally, because of the eccentric rotation of the rotor within the stator, a transmission mechanism, often a constant velocity joint, is used to convert the eccentric rotation of the rotor into a rotation concentric with the drill bit. The eccentricity of motion of the rotor may be affected by this transmission mechanism and cause additional wear in locations of the stator near the transmission mechanism. Furthermore, where the transmission mechanism is near a point of flexure in the drill string, such as at a bent sub, the eccentricity may be further increased.